The invention relates to a region based-light detector system, and, more particularly, the invention relates to a method and apparatus for improving the accuracy of a region-based light detector by incorporating a strobe lamp as a light source.
Various position detectors are known in the prior art for detecting and measuring light as a means for determining position of a light source or an object disposed between the light source and the position detectors. For example, a method of measuring the direction to a light source utilizing a xe2x80x9cQuadrant Light Detectorxe2x80x9d (or region-based detector) is known. One kind of Quadrant Light Detector is a xe2x80x9cQuad Hemispherical Detectorxe2x80x9d (QHD) that employs a series of reflective cavities and a mask, as disclosed by Ramer et al. in U.S. Pat. No. 5,705,804, herein incorporated in its entirety by reference. A second type of Quadrant Light Detector utilizes a circular mask spaced above a larger circular detector that is evenly divided into quadrants. Both types of quadrant light detector utilize a mask to occlude a fraction of the light incident on the device in relation to its direction.
To determine the direction to the source of light, the amount of light reaching each quadrant of the detector is measured. A calculation that utilizes the measurements will then provide the light source""s direction. An example of such a calculation is given by Ramer et al. The accuracy of the calculation will depend on the accuracy of the measurements. In turn, the measurement accuracy is limited by the intensity of the light signal reaching the light detectors and the noise level of the measurement circuitry, and is dependent on their ratio (SNR). However, in certain directions, some quadrants of the detector may receive only 1% or less of the light received if fully illuminated.
In operation, the measurement of direction is typically used to locate a source, an object, multiple objects, or the Quadrant Detector itself within a volume of space. For example, a single QHD would provide direction to an object, while two or more QHD""s would provide an object""s position through triangulation. Thus, the measurement accuracy would then limit the accuracy to which the location of the object could be known. Due to increased occlusion for extreme angles, this accuracy degrades as the signal strength decreases. In many applications, such as metrology, guidance, motion control, collision avoidance, or targeting, performance is heavily dependent on the measurement accuracy, such that for a specified performance level, a particular level of measurement accuracy is required. If the required level of measurement accuracy cannot be achieved, then the deployment of Quadrant Light Detector is not practical.
Nevertheless, the measurement accuracy of a Quadrant Light Detector can be improved by increasing the SNR of the measurement system. This can be accomplished either by increasing the amount of light signal received by the Quadrant Light Detector or reducing the noise present in the measurement circuit or both.
Typical light sources that are readily available include a light emitting diode (LED) or Laser diode. These sources can be modulated or pulsed to work in conjunction with electronic filter circuitry. The filter serves to reduce the effect of ambient light that appears as noise in the optical signal as well as electronic noise in the circuitry itself. An LED provides a small amount of optical power which when radiated into a large solid angle (a large angular region of space) severely limits the range at which a sufficient light signal is received by a typical Quadrant Detector.
A laser, by its principle of operation, produces a tightly collimated beam that radiates into an extremely small solid angle (a small angular region of space). The more intense collimated beam of the laser, or the beam from an LED with a collimating lens, could be scanned through a large solid angle to compensate for the small angle subtended by the beam. The more intense beam increases the signal received by the QHD. An array of collimated lasers or LEDs can also be constructed to partially illuminate a large solid angle. However, both a scanning system and an array are undesirable due to increased cost, mechanical complexity, and size. The scanning system suffers from a reduced response time since for the majority of time, the beam is not directed toward an object. The array suffers from an incomplete coverage of the detection volume. These same limitations are also present in the case of an LED or laser utilized as a source located in the detection volume and illuminating the Quadrant Detector directly.
Furthermore, since many applications impose both a required measurement accuracy and a minimum range of operation, LED and laser sources are limited. Namely, typical range of operations with scanned LED or laser sources are less than 10 meters and less than 1 meter with LED sources without a lens. This range is not sufficient for many applications in collision avoidance, guidance, motion control, targeting, and object detection. Thus, the result being that neither an LED nor a laser is desirable as a source for use in an accurate measurement apparatus that requires a sufficient range.
To illustrate, the QHD is sensitive to light that emanates from a roughly hemispherical volume of space that lies in front of the detector. This light either can be produced directly from a source located in the volume or could be reflected from an object located in the volume. The size of the volume wherein accurate measurements can be obtained will be determined by the range at which a sufficient optical signal is received to produce the required SNR. Typically, a fixed amount of noise will be present in the circuitry of the apparatus. When the light source is located inside the volume, it must emit a sufficient quantity of light in a direction toward the detector in order to produce a signal large enough to be accurately detected. The source must then radiate into a large solid angle when the direction to the detector is not known or it is impractical to mechanically direct the source. A sufficient quantity of light must be reflected from an object back to the Quadrant Detector to accurately measure the direction to the object. Since the location of the object is not initially known, the source must radiate into the large solid angle in which the Quadrant detector is sensitive. In both cases, the source employed must radiate into a large solid angle and must have sufficient power to produce the SNR required by the application.
Additionally, a number of other methods can be employed to increase the optical signal that reaches any given detector. For example, the signal can be increased by making the detector larger. However, an increased Quadrant Detector size is not desirable as it reduces the speed of the system while increasing noise. A larger detector would also increase the cost of the system and result in a larger apparatus.
Alternatively, a lens can be employed to collect light from a large area and focus it onto a detector. However, the principle of the Quadrant Detector prevents the use of a lens, since a Quadrant Detector measures the direction light is traveling while a lens alters this direction.
Alternatively, the signal can be increased by reducing the distance between the source and detector. While this alternative approach will improve the measurement accuracy, the reduced distance will also reduce the volume within which the Quadrant detector can function, i.e., a reduced range of operation. Thus, these methods are also not practical in increasing the optical signal received by a Quadrant Detector.
Therefore, a need exists in the art for an apparatus and method that improves the measurement accuracy of a Quadrant Detector, where the light source or objects are located at greater distances.
The present invention is a method and apparatus for improving the accuracy of a region-based light detector by incorporating a strobe lamp as a light source. The present apparatus allows the directional measurement properties of a Quadrant Detector to be exploited in greater number of applications where measurement accuracy and/or greater range of operation are required.
The present invention is an apparatus for producing sufficient illumination into large solid angles and generating accurate measurements of the optical signals received by a Quadrant Detector from sources or objects located more than a few meters from the detector. More specifically, the invention comprises a strobe lamp with associated optical filters, shields, lenses, power supply, and encoding trigger circuitry, a strobe pulse timing detector with associated electronic circuitry, a Quadrant Detector with associated detection, amplification, filtering, sampling, and analog to digital conversion circuitry, and an electronic signal processing and control circuit with an associated output device.
In operation, either the electronic control circuit triggers the lamp encoding trigger circuitry or the lamp encoding trigger circuitry triggers itself by means of an internal circuit. The trigger circuit initializes one or a timed series of arcs within the strobe tube, having the capacity to transmit data via the light signals produced. The lamp""s power supply circuitry stores energy to produce an extremely intense, short duration pulse of electric current through the gas contained in the strobe tube, resulting in an arc that produces an intense light. This light pulse is transmitted into a region of space determined by the shield and lenses associated with the lamp. An optical filter controls the wavelength band of the transmitted radiation to prevent interference with human vision or other undesired effects.
A Quadrant Detector responds to the light pulse or pulses that have traveled directly from the source or that have been reflected from an object. The path the light travels is determined by the application and controlled by the shielding. The light received by each quadrant of the quadrant detector is controlled by a mask that limits the light according to its direction of travel. The light entering each quadrant is detected by a photosensitive component that produces an electrical signal, which is a function of the light intensity. The light reaching the detector consists of light transmitted by the strobe lamp as well as ambient light.
An electronic circuit is associated with the detector. The circuit consists of a preamplifier, a gain controlled pulse amplifier and filter, a sample and hold circuit and an analog to digital converter for each of the detectors in the Quadrant Detector. The circuit amplifies the signal produced in the detectors by the brief, intense pulse from the strobe lamp while removing the relatively slowly varying portion of the signal produced by ambient light. The gain of the pulse amplifier is controlled to prevent saturation when a strong signal is present as occurs when an object is close to the detector. The level of gain is communicated to the signal processing circuit to scale the measurements. The strobe pulse timing circuit and discriminator detects the moment at which the radiation from the strobe is peaked. The pulse timing circuit discriminator determines if the magnitude of the pulse is sufficient for accurate measurement. If so, the pulse timing circuit then initiates the sampling of the received signal, when gated by the electronic control circuit, by triggering the sample and hold circuitry. The pulse timing circuit output is also directed to the electronic control circuit.
The electronic control circuit upon receipt of the peak timing signal initiates the analog to digital conversion circuitry. The electronic control circuit reads the result of the analog to digital conversions after such conversions are completed. The results are processed by a calculation that results in a direction to the received light expressed as a pair of angles in a spherical coordinate system. The control circuit transmits the results to an external device via the output device. The direct measurements could also be transmitted, without calculation of direction, for further processing by an external device.
The process of reception of light pulses may be repeated in a timed manner in order to decode a data transmission. Reception of multiple pulses with a predetermined timed sequence could be used as a means to distinguish a transmitted pulse train from the random pulse train produced by another bright arc source such as those produced during arc welding.
The intensity of the light pulse from the strobe lamp and the brevity of the pulse when combined with the pulse amplification, filtering, and timed sampling result in a measurement apparatus with the capability of improved accuracy when the strobe lamp or objects are located at large distances.